(1) Field of the Invention
This invention relates to a radiographic apparatus including a radiation source and a radiation detector. More particularly, the invention relates to a radiographic apparatus including a radiation source and a radiation detector which are both movable, the radiation detector having a radiation grid attached thereto.
(2) Description of the Related Art
Among radiographic apparatus for acquiring fluoroscopic images of objects under examination or patients, there is one constructed to emit cone-shaped radiation beams from a radiation source toward a patient, and detect with a flat panel detector (hereinafter abbreviated as FPD) radiation transmitted through the patient. Such a construction is shown in Cited Document 1. The radiation, when transmitted through the patient, becomes scattered inside the patient, resulting in scattered radiation incident on the FPD to affect the fluoroscopic image. This becomes a factor for worsening the contrast of the fluoroscopic images. In order to prevent the scattered radiation from reaching the FPD, as shown in FIG. 17, a radiation grid 71 may be provided to cover the radiation detecting plane of the FPD. The radiation grid 71 is constructed to have absorbing foil strips 72 arranged in a predetermined direction.
Strictly speaking, these absorbing foil strips 72 will also absorb direct radiation. Therefore, shadows of the absorbing foil strips 72 will fall on the FPD 74. The luminosity of portions of the fluoroscopic image corresponding to the detecting elements reflecting these shadows will become lower than that of portions corresponding to the detecting elements free of the shadows. This results in striped false images appearing on the fluoroscopic image.
Such false images appearing on the fluoroscopic image are obstructive to diagnosis of a site of interest. In a known method of removing these false images, the shadows of the radiation grid 71 are once imaged and the false images are removed based on this. Such method projects the shadows of the radiation grid 71 onto the FPD 74 without a patient placed between the radiation source and FPD 74, and forms a pattern of the radiation grid shadows. When actually removing the false images, the pattern of the radiation grid shadows is first increased or decreased in size, moved and rotated. Then, the false images are removed by superimposing the pattern on the fluoroscopic image of the patient (see Japanese Unexamined Patent Publication No. 2002-336220, for example).
However, the conventional radiographic apparatus has the following drawback. The conventional practice of removing the false images due to the shadows of the radiation grid 71 is premised on that the absorbing foil strips 72 of the radiation grid 71 are perfectly linear. Actually, the absorbing foil strips 72 of the radiation grid 71 are bent to meander as shown in FIG. 18A, or are twisted as shown in FIG. 18B. Such bending or twisting is slight and does not seriously block passage of direct radiation, but is important in the process of removing the false images.
Variations in the direction of radiation irradiating the absorbing foil strips 72, if the latter are bent or twisted, will also vary the shapes of the shadows. That is, in the radiographic apparatus, the shapes of the shadows falling on the FPD will vary with variations in the positional relationship between the radiation source, radiation grid 71, and FPD. Then, the pattern of the false images will also change. According to the conventional construction, it is impossible to foresee variations in the pattern of the false images complicated by bending or twisting of the absorbing foil strips 72. Therefore, the false images superimposed on the fluoroscopic image cannot be removed sufficiently.